1. Field of the Invention
This invention relates to means for detecting magnetisible electrically conductive particles in a fluid medium and is of particular use for detecting metal particles in the lubricant of an engine or a mechanical transmission system thus to give warning of malfunction of the system.
Preferred embodiments of apparatus according to the invention not only detect metal particles but also provide a measure of the concentration of such particles.
The invention will hereinafter be described with particular reference to its use as a warning apparatus for detecting contamination in the lubricant of the transmissions used in heavy agricultural and earth moving equipment to transmit mechanical energy from the engine or motor to wheels or to other means to be driven. However, it will be understood that the invention is not limited to applications involving vehicle transmission systems, or to detection of metal contamination in lubricants.
Transmissions normally utilized in heavy machinery either use gears which are encased in a gear box or utilize hydraulic rams which are housed in an appropriate housing. Gear boxes are lubricated by an appropriate lubricating fluid such as a mineral oil which is usually contained in a reservoir in the base portion of the gear box. In hydraulic transmissions there is a fluid reservoir which acts as the fluid source and there can be gearing mechanisms involved in operation of a hydraulic fluid pump.
When, for example, gearing teeth of a gear box are reaching a worn stage and or not meshing in the manner for which they are designed, flakes or particles of metal are passed into the lubricating fluid or hydraulic fluid and thereby normally travel to the fluid reservoir. After a period of time these metal particles build up in the fluid reservoir and this is normally recognized to be indicative that appropriate repairs should be initiated to the transmission. If the symptoms are not recognized at an early stage, and repairs are not effected, then damage can subsequently be caused which is extremely expensive to repair and which may require replacement of major components, or may even require the substitution of a new gear box or of a new transmission. Moreover, if appropriate repairs are not effected at an early stage the motor or engine may be adversely affected in addition to the transmission system. On the other hand, if the build-up of metal particles in the fluid reservoir is recognized at an early stage, corrective repairs may be undertaken which are usually relatively far less costly.
Particularly in the case of heavy earth moving or agricultural equipment in which the driver of the vehicle may be housed in a soundproof cabin the driver may not hear sounds coming from the transmission which are also normally indicative of faults such as worn out gear teeth or misalignments which result in the deposition of metal particles in the fluid reservoir. As a result he may run the transmission to a point which will result in major damage to the transmission and often to the engine of the vehicle.
Similarly with engine and transmission bearings it is normal for very fine particles of metal to be deposited from the bearing metals into the lubricant. However, an excessive rate of such deposition is an early indication of impending bearing failure. Particles of the type originating from wear of bearings are typically of a much smaller size than the particles and flakes oringinating from the wear of gears.
2. Description of the Prior Art
A magnetic chip guage is described in U.S. Pat. No. 3,373,352. It comprises a magnet sheathed by a wire coil of which adjacent portions of successive turns may be short circuited by ferrous particles attracted by the magnet. Circuit means are provided to detect changes in the resistance exhibited between the ends of the coil.
A major disadvantage of such detectors is that the sensitivity of the apparatus is dependent on where particles impinge on the coil to cause a short circuit. If two adjacent turns are short circuited by a metal chip, a second chip impinging adjacent the first and effectively shorting the same two turns has a small effect on the resistance of the coil in comparison with an identical chip impinging elsewhere on the coil to short circuit two different turns.
Moreover, since the magnetic field is non uniform and is concentrated at or near one end of the coil, particles tend to accumulate preferentially in a small region along the coil axial direction. That is to say in a manner providing the least sensitivity. While it might be expected that the sensitivity could be improved by extending the coil axially to provide a large number of turns in relation to coil circumference, in practice that merely aggravates the problem created by the tendancy of particles to concentrate at one end and reduces sensitivity by virtue of accumulation of most particles over a few turns of a longer coil.
In other devices for example those proposed in U.S. Pat. No. 2,462,715 and U.S. Pat. No. 4,008,464, metal particles attracted themselves are used to close a normally open circuit between two electrodes of which one is electrically insulated from the other.
U.S. Pat. No. 2,462,715 is particularly directed for use in aircraft engines and relates to a metal plug which may be screwed into a port opening into a part of an aircraft engine. It is characterized as having two bar magnets projecting as fingers upwardly from one end of the plug into the engine, one magnet having a north pole and the other a south pole both pole faces being substantially coplanar and in contact with the lubricant fluid. The magnets are spaced apart, and electrically insulated, one from the other. In use, metal particles in the fluid attracted to one or the other magnetic pole eventually form a bridge spanning the magnetic poles and providing a conductive path which closes a normally open circuit to light a warning light.
U.S. Pat. No. 4,008,464 describes a plug in which an insulated support member projects into the lubricant. Two annular magnets sleeve the support. The magnets are spaced apart in the fluid and when a bridge of metal particles spans from one magnet to the other, a normally open circuit between electrodes positioned adjacent each magnet is closed.
The prior art detectors suffer from a number of disadvantages.
Firstly, the detectors are not as sensitive as is desirable, that is to say the threshold quantity of metal required to trigger a warning is too high and consequently the detectors often fail to give a positive warning at a sufficiently early time during the development of a fault condition, at which time preventative repair could be undertaken at relatively low expense. In fact during the period in which metal particles are building up in the lubricant and prior to such devices giving a warning the presence of metal particles may itself be deleterious to the transmission system. For reasons which will later be apparent, the sensitivity of those detectors is, in practice inherently limited.
Secondly the quantity of metal required to trigger a warning tends to be that non reproducible, that is to say a quantity which will trigger alarm system in one experiment will not trigger an alarm in a second experiment. Efforts to increase the sensitivity of those devices have been found adversely to affect reproducibility.
Thirdly, the detectors only distinguish between a condition in which there are insufficient metal particles to bridge the electrode gap and a condition in which the gap is bridged. The detectors do not provide an indication of the concentration of particles or the rate of build-up of particles, both of which are important diagnostic aids.
Because of the above disadvantages prior art detectors have been limited in use to the detecting comparatively large metal fragments and have generally not been of use to detect, for example, excessive wear of bearings.